A method of measuring particle sizes from the scattered light spectrum using the optical mixing spectroscopy [1] is currently used worldwide and in Russia [2]. FIG. 1 shows a conventional optical arrangement for measuring particle sizes by optical mixing spectroscopy techniques, where 1 is an argon, He—Ne or semiconductor laser; 2—polarizers; 3—objectives or lenses; 5—an emulsion cell; Θ—the scattering angle; 4—an aperture diaphragm; 8—a diaphragm in front of a light detector; 9—a light detector; 10—a correlator or spectrum analyzer; 6—a homo- or heterodyning beam; 7—a diaphragm.
A focused beam of a laser 1 propagates through a polarizer 2, a focusing lens 3, a cell 5 with a sample under investigation, and the light scattered at angle Θ is provided to a light detector 9 by an objective 3 which creates, on a light detector cathode, an image of the laser beam in the cell. A small scattered volume is cut out from the beam by the image of diaphragm 8 and bounded by the laser beam diameter and the image of diaphragm 8 on the beam. An auxiliary reference laser beam 6 at the same frequency or frequency-shifted may be introduced in the scheme to provide optical homodyning or heterodyning.
Beats of light frequency components give rise to intensity fluctuations in a square-law light detector, the fluctuations repeating themselves in the detector current and being analyzed by a correlator or spectrum analyzer. Spectrum of the light scattered by monodisperse particles has a Lorentzian shape with halfwidth Γ, and the time correlation function of the field of such light is exponent. The correlator analyzes the photocurrent that repeats the light intensity, rather than the light field. The intensity correlation function is an exponent with a decay rate 2Γ on a “substrate” of noncoherent background. At ideal spatial coherence of the light field, the amplitude of the correlation function exponent is equal to the background [1]. The spectrum analyzer or correlator determines spectral scattering line width Γ or correlation function coherence time τc=1/Γ, and particle radius r is calculated therefrom:
                                                                        Γ                =                                                      q                    2                                    ⁢                  D                                                                                                        =                                                      KT                                          6                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      η                      ⁢                                                                                          ⁢                      r                                                        ⁢                                      q                    2                                                                                      ⁢                                  ⁢        or        ⁢                                  ⁢                  r          =                                    KT                              6                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                η                ⁢                                                                  ⁢                Γ                                      ⁢                                          (                                                                            4                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      n                                        λ                                    ⁢                  sin                  ⁢                                      Θ                    2                                                  )                            2                                                          (        1        )            
where q is the scattering vector {right arrow over (q)}={right arrow over (k)}s−{right arrow over (k)}L, {right arrow over (k)}s, {right arrow over (k)}L are wave vectors of the scattered and incident light,
                  q              =                            4          ⁢                                          ⁢          π          ⁢                                          ⁢          n                λ            ⁢      sin      ⁢              Θ        2              ,n is the refractive index of the medium; λ is the wavelength of incident light; Γ is the scattering angle; K is the Boltzmann constant; T is the absolute temperature; η is the shear viscosity; D is the diffusion coefficient of particles.
Where a suspension or emulsion includes particles of several sizes the time correlation function is a sum of several exponents whose amplitudes are proportional to squared intensity of light scattering on particles of respective size. In this case, spectrum is the Lorentzian sum.
The important point is that to conduct spectral or correlation analysis of intensity beats, the light entering at the light detector must be spatially coherent, otherwise the intensity beats at various small areas of the light detector will be out of phase and compensate each other, this making the analysis thereof impossible. In the conventional arrangement, the spatial coherence conditions are attained owing to the use of a small source (scattering volume) with efficient diameter ds and the inclusion, in the optical scheme, of a small aperture diaphragm 7 (FIG. 1) having diameter da. For the source and a round diaphragm, the condition of presence of spatial coherence (or, as is generally referred to, detection of a single coherence area) is expressed by formula [1]:
                                          d            a                    ⁢                      d            s                          ≤                              1.22            ⁢                                                  ⁢            λ            ⁢                                                  ⁢            L                    π                ≈                              λ            ⁢                                                  ⁢            L                    2.57                                    (        2        )            
where L is the distance between the source and the aperture diaphragm.
Practical application of the conventional arrangement for particle sizing in industry is prevented by the necessity to place a sample into a special optical cell, and due to the presence of dust and coarse aggregate formations both in raw stock and products.
The former problem has been partially solved (only for pipelines!) in Great Britain through the provision of a quite complex bypass line including a kind of an optical cell [3]. The situation is still worse with the problem of dust.
First, coarse dust particles scatter light more intensely than fine colloid particles of interest. Measurements are possible only due to a high concentration of the particles measured, but distortions are still present. To overcome this effect, complex mathematical processing of correlation functions and separation of the resulting exponents by Tikhonov regularization methods can be employed. However, the impact of dust on the measurement results is not restricted by the additional scattering only.
In a conventional optical arrangement, the scattering volume is defined by intersection between an exciting laser beam (Ø˜0.2 mm) and a photomultiplier field of view (Ø˜0.2 mm at ds=da) and amounts to Vsc≈0.8·10−5 cm.
The so small scattering volume either contains a few dust particles or is completely free of them. For this reason, first, the intensity fluctuations of dust-scattered light have a non-Gaussian statistics, and the size distribution of all particles, including the dust ones, cannot be determined from the resulting correlation function (or spectrum). Second and even more important, with such statistics of dust-scattered light the background constant component of the resulting correlation function changes; exponent expansion of monotonically decreasing portion of the function becomes inadequate, so the results even may have nothing to do with sizes of actually important particles and define only sizes of dust particles and the relationship between the concentration thereof and the scattering volume.
And finally, in many cases nanoemulsions and particularly microemulsions used in industry are virtually opaque due to strong light scattering, so to measure particle sizes therein the sample should be placed in a special thin (about 5÷100 μm) flat cell, this making real-time measurements and on-line process control impracticable.
Methods of using, in light scattering spectroscopy, fiber optic probes that are placed directly in the medium under investigation are disclosed in [4,5]. But these devices fail to measure correlation functions of scattered light because an optical fiber per se doesn't provide spatial coherence conditions. It is precisely this fact that caused failures of first attempts to use optical fibers for investigating narrow spectral lines of scattered light and measuring particle sizes therefrom.
Improvements in the conventional optical arrangement with optical fibers were initially reduced to the use of a conventional optical system; the optical fiber only transmitted the exciting light from a laser to a cell and/or from an aperture diaphragm to a light detector [6,7]. Then optical fiber “testers” (a kind of a probe) have appeared comprising a lens between the “tester” and the scattering volume [8,9] or an integrated optics at fiber ends [10,11] or both [12]. In [11] it is implicitly suggested that spatial coherence conditions should be provided by the use of single mode optical fibers. In [10] spatial coherence conditions are attained by intersection of a light beam with a small diameter field of view. [12] does not deal with this issue. In an apparatus disclosed in [13] for measurements in turbid media, light is introduced into the volume under investigation, and scattered light is output through the same single mode optical fiber; and a “coupler” (a device for beams-sharpening in a single optical fiber and dividing the light from the single optical fiber between several ones) is used. Multiple scattering should be eliminated in [13] by the use of a light source with a short coherence length.
Most closely related to the present invention is a method and an apparatus for measuring particle sizes in a liquid by optical mixing spectroscopy technique using optical fibers in the presence of multiple light scattering, as disclosed in [14]. The method comprises introducing light from a low coherence source into a volume under investigation and outputting scattered light through the same single mode optical fiber, and using a “coupler” (means for beams-sharpening in a single optical fiber and dividing the light from the single optical fiber between several ones), in which the source light is added to the scattered light for further heterodyning thereof. It is supposed that spatial coherence conditions are observed owing to the use of single mode optical fibers. Multiple scattering should be eliminated by the use of a light source with a short coherence length. This supposedly enables the correlation function of light scattered in a small volume only to be obtained using heterodyning.
The prior art method has the following disadvantages: due to the use of a small coherence volume, light that enters the light detector from the remaining volume of the fiber field of view, even if does not produce a correlation function distorted by multiple scattering, generates an enormous noncoherent background. Then, first, the correlation function will have an extremely small amplitude as compared to the background (hence a low precision), and second, due to a small efficient coherent scattering volume, when dust particles are present, the correlation function background will be distorted due to non-Gaussian statistics thereof and this will give rise to errors in the obtained correlation function processing, so measurements of particle radii in turbid media are impracticable. In addition, the obligatory application of single mode fibers involves a complicated adjustment of the system and its sensitivity to mechanical disturbances.
The present invention is aimed to:
extend functionality owing to the ability of measurements in turbid media in which multiple light scattering is present;
improve precision, reliability and immunity to interference (from dust) of particle size measurements in micro- and nanoemulsions, colloidal solutions and suspensions of particles in actual industrial conditions, without the need for placing the sample into a special cell and for special adjustment;
provide tolerance to mechanical disturbances.